The present invention relates to a receiver which demodulates an FSK (Frequency Shift Keying) modulated signal, and particularly to a circuit which controls a reception signal.
An FSK modulation system is a sort of frequency modulation system and is of a system that communicates a digital signal through an analog transmission line. This has widely been adopted for a portable receiver or receiving device such as a pager, beeper or the like because the digital signal can be transmitted/received in a relatively simple configuration.
Since, however, reception performance is degraded significantly when the frequency being present in transmission deviates, the receiving side needs to perform a correction for controlling a receive or reception frequency and allowing it to approach a transmit or transmission frequency.
A frequency control circuit for detecting a deviation of a phase plane to correct the frequency has been disclosed in each of patent documents 1 (Japanese Unexamined Patent Publication No. Hei 11(1999)-313117) and 2 (Japanese Patent No. 3178268). A frequency control circuit for correcting the frequency according to the amplitude of a frequency-voltage conversion signal has been disclosed in a patent document 3 (Japanese Patent No. 3070733). Further, a frequency control circuit for correcting the frequency by feedback processing of a digital signal has been disclosed in a patent document 4 (Japanese Unexamined Patent Publication No. Hei 10(1998)-257110).
When a deviation in phase plane is detected to correct the frequency in each of these conventional frequency control circuits, there is a need to decompose a receive or reception signal into orthogonal components in order to carry out it by digital signal processing and provide correction circuits on a component-by-component basis. That is, a problem arises in that since the same circuits of two systems are required, the mounting area of each circuit cannot be reduced.
A problem also arises in that since an AFC (Automatic Frequency Control) circuit comprised of an analog circuit exists where the amplitude of the frequency voltage conversion signal is used, there is a possibility that parameters of circuit elements will vary due to temperatures, variations with time and the like, thus causing an unexpected operation.